Currently deployed systems and methods developed for transmission of signals through copper twisted pairs were initially dedicated to low-speed (64 KBits/sec) telephone services. To provide telephone service, the US territory is divided into a plurality of service areas known as Customer Service Areas (CSAs) of specific dimensions. For example, with 24-gauge twisted pair wiring, maximum distance of 4 miles between a Central Office (CO) and customer premises is typical for the U.S. This distance limitation is defined by signal attenuation and channel-to-channel crosstalk in twisted pair cables.
Before Internet development, an idea of transmitting video over twisted pairs was extensively explored. Recently, twisted pair telephone cables were utilized for Internet connections with the bit rate of the order of 1 MBits/sec and faster. DSL technology and its versions (ADSL, VDSL) were developed to meet technical requirements of different applications. DSL modems became conventional devices for Internet connection used by businesses and households in the USA and other countries. However technical specifications of existing copper networks originally formulated for narrow band telephone connections create technical problems and constrains for Internet applications.
Twisted pair cables are characterized by frequency dependent power loss, phase delay and interference noise, especially pronounced at high frequencies. Substantial power loss (attenuation) and crosstalk accumulation as a function of frequency is observed even at low frequencies of several KHz, and above 600 KHz signal power level becomes lower than crosstalk making signal transmission difficult [for example, J. A. C. Bingham, “ADSL, VDSL, and Multicarrier Modulation”, John Wiley and Sons, Inc., 2000, pp.29, 48, 49]. To take care of signal power loss and distortions, Discrete Multi-Tone (DMI) transmission format was developed initially for voice service (for example, U.S. Pat. No. 4,731,816), and later applied to DSL transmission (for example, U.S. Pat. No. 5,673,290). In DMT format, spectrum is sliced in many narrow slots, with attenuation and dispersion almost constant within the slot. In each slot, a carrier frequency source is provided. The presently accepted and standardized Asymmetric Digital Subscriber Line transmits data using DMT scheme with 256 tones (frequency slots) each 4.3125 kHz wide, full frequency range being 1.104 MHz Bit stream of rate b is converted into several parallel symbols which are applied to modulate a discrete set of tones, then Fourier-transformed into time-domain samples, passed through P/S converter and sent through the transmission line as a time-dependent waveform. Quadrature Amplitude Modulation (QAM) is applied to the carrier wave in each frequency slot; both number of bits and transmitted power may be optimized depending on carrier wave attenuation and phase shift in a given slot. On the receiving end, signal amplitude and phase in each frequency slot is individually equalized, and other procedures are applied in the inverse order.
Though the improvements achieved by DMT systems are very substantial high frequency services provided in the field commonly does not cover more than 50% of CSA. In all practical applications, bandwidth was “traded” for distance. Today, ADSL service (1.5 MBits/sec) may be delivered over 12,000 ft, which is substantially less than maximum distance across CSA. Limitations of copper cables are even more pronounced for bit rates higher than 1.5 MBits/sec. A wide variety of business applications requires transmission rates of 25 MBits/sec or 50 MBits/sec. These kind of signals may be transmitted through twisted pairs only at very short distances (less than 1,000 ft at 100 Mbits/sec). To deliver high bit rate service at longer distance, inverse multiplexing (IMX) technology was developed, where the high-bit rate signal is demultiplexed into lower bit rate traffic streams, and low bit rate traffic streams are transmitted over several independent twisted pairs. Thus, IMX transmission of ˜50 Mbit/s through 18,000 ft line was achieved using 24 to 48 pairs. Details of this transmission technology are described, for example, in U.S. Pat. No. 6,687,288. IMX technology does not upgrade individual pair performance directly.
The major technical problem limiting wider deployment of broadband services is pair-to-pair interaction in telephone cables. The twisted pair is an open circuit, and interaction of the pair's electromagnetic field with other circuits causes both power attenuation and crosstalk. Two types of crosstalk dominate, far-end crosstalk (FEXT) caused by interaction of traffic downstreams in adjacent pairs, and near-end crosstalk (NEXT), caused by interaction of upstream and downstream traffics. General approach called Dynamic Spectral Management (DSM) was developed to improve performance of individual twisted pairs by managing traffic through several pairs together. The simplest DSM approach is to balance spectral power in adjacent pairs to optimize performance of the whole cable rather than individual pair. An efficient algorithm of Iterative Water Filling [W. Yu, W. Rhee, J. Cioffi and S. Boyd, “Iterative Water-filling for the Vector Multiple Access Channel,” ANSI Contribution T1E1.4/2001-200R4, November 2001, Greensboro, N.C.] was developed for power management in multiple DMT systems to equalize signal/noise on the same frequency tones in multiple systems.
Another DSM approach is known as vectoring, or crosstalk-free transmission through twisted pair cables. The general idea of vectoring is to pre-distort the transmitted signal so that by the time the signal reaches the receiver, this pre-distortion is fully compensated by pair-to pair interaction. Complicated transmission environment, with pairs strongly interacting with each other inside the cable, makes vectoring an attractive but technically difficult problem. The cable of n twisted pairs may be presented by a matrix equation [S. Verdu, Multiuser Detection, Cambridge University Press, UK, 1998; G. Ginis and J. M. Cioffi, “Vectored-DMT: A FEXT Canceling Modulation Scheme for Coordinating Users,” Proceedings of IEEE International Conference on Communications 2001, Vol. 1, Helsinki, Finland, pp. 305-309, June 2001; J. M. Cioffi, EE 379c textbook, “Digital Transmission Theory, Volume I,” http://www.stanford.edu/class/ee379c/]:Y(f)=H(f)·X(f)+N(f)  (1)where H(f) is a n×n matrix of channel transfer functions, X(f) is a “vector” of n inputs, N(f) is noise, and Y(f) is a vector of n channel outputs. Off-diagonal matrix elements of H represent mutual crosstalk between each couple of interacting pairs.Ideal vectoring is described by the following equation:Z=WY=BX+E  (2)where matrix W causes the channel output Z=WY to appear free of crosstalk with B having all off-diagonal elements equal zero and the error matrix E being “white” noise. Any practical approach to implement the system described by Eq. (2) implies resolving two matrix equations:
                              (                                                                      Z                  1                                                                                                      Z                  2                                                                                    …                                                                                      Z                  n                                                              )                =                              (                                                                                H                    11                                                                                        H                    12                                                                    …                                                                      H                                          1                      ⁢                      n                                                                                                                                                                                                                                H                    22                                                                    …                                                                      H                                          2                      ⁢                      n                                                                                                                                                                                                                                                                                                    …                                                  …                                                                                                                                                                                                                                                                                                                                                                          H                    nn                                                                        )                    ⁢                      (                                                                                X                    1                                                                                                                    X                    2                                                                                                …                                                                                                  X                    n                                                                        )                                              (        3        )            where matrix elements Wik are defined from the equation:
                              (                                                                      X                  1                                                                                                      X                  2                                                                                    …                                                                                      X                  n                                                              )                =                              (                                                                                W                    11                                                                                        W                    12                                                                    …                                                                      W                                          1                      ⁢                      n                                                                                                                                                                                                                                W                    22                                                                    …                                                                      W                                          2                      ⁢                      n                                                                                                                                                                                                                                                                                                    …                                                  …                                                                                                                                                                                                                                                                                                                                                                          W                    nn                                                                        )                    ⁢                      (                                                                                Z                    1                                                                                                                    Z                    2                                                                                                …                                                                                                  Z                    n                                                                        )                                              (        4        )            Eq. (3) and (4) are equivalent to Eq. (1) and (2) without noise contributions. To determine n(n−1)/2 matrix elements Wik for n pairs, equations (3) and (4) should be applied to at least (n−1)/2 vectors X, to provide a complete system of linear equations. If n=10, the system of 45 equations has to be resolved, each matrix element Wik being expressed as a sum of multiple products of matrix elements Hik from Eq. (3). Even if the numerical difficulties are resolved the statistical errors in Hik when transferred into Wik values will accumulate. For reference purposes, FIG. 1 illustrates relative values of averaged crosstalk powers detected in one pair (No. 17 in FIG. 1) of the 25-pair bundle from the surrounding pairs. Obviously, adjacent pairs (No. 16, 18 and 19) give the biggest contributions. However all other pairs, if coincide in phase, give a contribution comparable to pairs 16, 18 or 19. Thus, vectoring of pair No.17 might need mutual consideration of almost all other pairs in the bundle. Within approach of Eq. (3, 4), a system of 276 linear equations with 276 unknown Wik has to be resolved with accuracy determined by real and imaginary parts of many contributions of the type
      ∑    k                  ⁢          ⁢            ∏              k        ′                                  ⁢                  ⁢                  H        ik            ⁢                        H                                    i              ′                        ⁢                          k              ′                                      .            With the values Hik measured at opposite sides of all pairs, with multiple equipment reconnections, reliable definition of Wik looks problematic for any appreciable number of pairs n>>1.
No commercial system based on DSM is available at the present time but some models describing vectoring of small number of pairs (3-4) are mentioned in literature [for example, “Vectoring Techniques for Multi-line 10MDSL Systems”, T1E1.4/2002-196 Voyan Technology Contribution to Committee T1—Telecommunications Working Group T1E1.4 (DSL Access), Denver, Colo., Aug. 19-23, 2002]. Calculations demonstrate that vectoring may improve individual pair performance by several times; however, small number of pairs involved and difficulty of practical implementation limit perspectives of this method of vectoring. For these reasons, different iterative procedures were developed [K. W. Cheong, W. J. Choi and J. M. Cioffi, “Multiuser Soft Interference Canceller via Iterative Decoding for DSL Applications,” IEEE Journal on Selected Areas in Communications, Vol. 19, No. 2, February 2002] to improve performance by averaging crosstalk with many pairs.
None of the approaches discussed above offers vast improvement of broadband transmission via telephone cables by crosstalk reduction. In the present invention, system and method is provided to improve individual pair transmission by canceling crosstalk, using the phenomena of phase conjugation.